1. Field of the Invention
The present invention relates to drive equipment supplied with direct current, with a DC motor with a series field winding and with a controllable semi-conductor rectifier, arranged in parallel with the field winding and working in pulse operation, for variable shunting of the field winding.
2. The Prior Art
Equipment of this kind is previously known, for example, through the French Pat. No. 1,373,565. This describes a DC series motor, the field winding of which is parallel-connected to a controllable semi-conductor rectifier (a thyristor with a turn-off circuit). In the field weakening range, that is at relatively high speed, the thyristor is controlled so that it works in pulse operation, thus obtaining the desirable variable shunting of the field winding.
During the intervals when the thyristor is conducting, the voltage across the field winding is zero. During the intervals when the thyristor is non-conducting, the whole motor current flows through the field winding and the voltage across the winding will then become relatively high, in a typical case some ten or twenty volts. The voltage over the winding thus varies periodically between zero and the last-mentioned value and contains a strong alternating voltage component. It has been found that this component causes considerable difficulties in commutation for the motor.